1. Field of the Invention
This invention relates to an infrared detector and more particularly to an infrared detector of an infrared sensing array of a bolometer, thermo pile, or pyroelectric system requiring a vacuum airtight property for a light reception part and an infrared detector of an infrared sensing array of a bolometer system requiring temperature stabilization of a light reception part.
2. Description of the Related Art
In recent years, the use of optical machines with infrared rays has been increasing and has been made in night monitor, temperature measurement, etc. With the increasing range of uses for the optical machines with infrared rays, a demand is made for developing an inexpensive infrared detector made of a thermal detector of a bolometer, thermo pile, or pyroelectric system. For example, an infrared sensing array of a bolometer system with detector pixels suspended in bridge structures as described in Japanese Patent Laid-Open No. Hei 2-196929 or 4-500437, an infrared sensing array of a thermo pile system described on pages 450-459 of SPIE Proceedings Vol. 2269 Infrared Technology XX, (1994), or an infrared sensing array of a pyroelectric system described in Japanese Patent Laid-Open No. Hei 7-243908 is an infrared detection element comprising infrared ray sensing pixels formed like an array on a silicon substrate. The elements enable infrared rays to be sensed as images. The thermal detectors, which do not require element cooling as compared with infrared image detectors of quantum type, have the advantage that they can provide infrared detectors at low costs.
However, the thermal detector converts incident infrared rays absorbed at a light reception part into temperature change of the light reception part and detects it as a signal. Thus, the thermal insulation property of the light reception part needs to be enhanced to provide a highly sensitive detector. In the prior art, to obtain a high thermal insulation property, pixels of a light reception part are suspended in bridge structures and the whole substrate forming a detection element is placed in a vacuum vessel. A vacuum vessel, also called a vacuum package, is provided, for example, as shown in FIG. 14: An infrared detection element 20 is bonded to a ceramic stem (base) 15 and a cap 16 with an infrared transmission window 4 is placed on the front of the detection element 20 and is bonded to the stem 15 in airtight relation. The cap 16 is evacuated of air through an exhaust pipe 17 attached to the cap 16 and the end face of the exhaust pipe 17 is sealed for finally providing a vacuum vessel. A signal of the detection element 20 is connected to a signal pin 19 penetrating an element wiring pad and the stem 15 by wire bond 18, whereby the signal is taken out to the outside of the vessel.
In the method of using such a vacuum vessel (package) to hold the thermal insulation property of a detection element, it is difficult to miniaturize a detector, needless to say, because the whole detector dimensions are determined by the vacuum vessel no matter how small the detection element is made. The detection element mounting process with the vacuum vessel not only is intricate, but also requires expensive parts such as the ceramic stem and large transmission window; it does not necessarily provide a detector at low costs.
The thermal detector of a bolometer system converts absolute temperature change of pixels made by irradiation with infrared rays from the outside into resistance change and reads it. Thus, the temperature of the detector itself needs to be stable independently of change in the ambient temperature of the detector. In the prior art, for example, as shown in FIG. 15, an infrared detection element 20 and a thermoelectric thermo module 22 are bonded and are fixed in a vessel made up of a cap 16 provided with an infrared transmission window 4 and a stem 15 in a similar manner to that shown in FIG. 14, and the vessel is evacuated of air and is sealed. Further, a temperature sensor 23 is located in the proximity of the detection element 20 for sensing the temperature of the detection element 20, and a signal from the temperature sensor 23 is taken out to the outside by a signal pin 19 penetrating the stem 15 and is connected to a temperature controller 24 for controlling the thermoelectric thermo module 22. The temperature controller 24 adjusts current into the thermo module 22 so that the temperature of the detection element 20 holds a predetermined value in response to the signal from the temperature sensor 23.
The thermoelectric thermo module, which generally uses the Peltier effect, can absorb or generate heat according to the amount and direction of energized current for holding the detection element temperature to a predetermined value under the current control of the temperature controller. However, additional circuits and units such as the temperature sensor and the temperature controller for controlling the thermoelectric thermo module are required, causing a hindrance to low costs.